We are accustomed to reading and writing or drawing on paper. Paper is convenient and is readily available but it can only be used once; after use, it is promptly discarded. Discarding paper is a huge problem world-wide at a significant cost and impact on the environment. Recycling it has been a growing business. Using more modern technology, brought on by computers with touch screens and powerful software, we can write on computer screens and the image can be saved or discarded without waste. An example is the iPad® of Apple Inc. However, we have not had the privilege of something that truly imitates paper but does not need to be discarded after use.
Photochromic paper has been disclosed for paper replacement that can be erased and reused; see for example, U.S. Patent Application Publication 2010/0216635 by Xerox. A problem with photochromic paper, however, is that the image is transient, being erased by ambient and sun light. It also requires a special optical stylus or masked light to create an image.
U.S. Pat. No. 6,104,448 discloses a thin film of cholesteric liquid crystal material sandwiched between two thin sheets of plastic used as a writing surface. The light pressure of a pointed stylus such as a pencil can be used to write or draw an image on the tablet. A unique feature of the tablet is that the image remains indefinitely under sun and ambient light and even elevated temperatures which can exceed 50° C. With transparent electrodes on the plastic substrates and attached drive electronics, the image traced on the writing pad can be written and electrically erased and rewritten with a different image. A writing pad including a cholesteric liquid crystal layer sandwiched between substrates and in contact with electrodes, which is erased by attached drive electronics, has been recently commercialized under the name Boogie Board™ by Kent Displays, Inc.
More recently, full-color writing tablets have been proposed (see U.S. Patent Application Publications 2009/0033811 and 2009/0096942), which are incorporated herein by reference in their entireties. The patent applications disclose, among other things, writing pads in which two or more cholesteric liquid crystal layers that reflect different primary colors are stacked on top of one another. With a stylus one can draw or write in a different primary color. Colors are changed using attached drive electronics. The primary colors can be mixed depending upon the pressure of the pen to create many different colors. This provides the user of a writing pad to create artwork in full color.
As disclosed in U.S. Pat. No. 6,104,448 polymer networks can be incorporated into the cholesteric liquid crystal layer. U.S. Patent Application Publication 2009/0033811 further discloses how the polymer networks can be used to control the pressure sensitivity of the stylus in tracing an image. Published U.S. patent application Pub No. 2012/0099030, which is incorporated herein by reference in its entirety, discloses the use of a spacer network where the density and placement of the spacers control the pressure sensitivity.